Inhalation of airborne particles (“aerosols”) causes a wide range of negative health effects. The toxicity of aerosols depends on their physical form and size. Long-term and short-term exposure to particulate air pollution has been linked to increased cardiopulmonary mortality, lung cancer, asthma, heart failure, and other cardiovascular diseases. The World Health Organization (WHO) estimates that exposure to outdoor air pollution is responsible for 1.4% of global mortalities and 2% of cardiopulmonary disease. This well-proven link has led to the development of exposure limits and air pollution regulations by organizations including the US Environmental Protection Agency and the WHO, who recommend that long-term exposure to PM2.5 (airborne particulate matter of diameter<2.5 μm) not exceed 15 μg/m3 and 10 μg/m3 respectively.
Different applications of aerosol exposure monitoring present different needs. The WHO recommends monitoring PM2.5 exposure over several years in fixed-site monitors, particularly throughout metropolitan areas. Occupational health studies may require monitoring numerous specific individuals over long time periods.
Current particle monitoring technology is typically prohibitively expensive for performing high resolution geographic monitoring and too large and cumbersome to be carried conveniently by individuals. Continuous personal exposure monitoring and geographically-distributed environmental monitoring applications require a sensor that is smaller and lower in cost than existing devices and at least capable of measuring the levels of PM2.5.
Currently available commercial technologies used in environmental monitoring and occupational health applications include gravimetric samplers, in which a known volume of sample air is drawn through a filter that is weighed before and after the measurement. Gravimetric sampling may be used in combination with impactors and cyclones, which select for particle size by causing sudden changes of direction in flow streamlines such that smaller particles continue with the flow and larger ones cross the streamlines to impact on a surface or pass through an orifice. Gravimetric techniques have the advantage of simplicity and of collecting sample for later chemical analysis; however, they require careful and time-consuming pre- and post-weighing of the filter and do not allow the real-time measurement of data.
A modern gravimetric technique for monitoring for airborne particles uses a tamper element oscillating microbalance (TEOM). A TEOM measures aerosol mass directly and in real-time, but is expensive and, currently, only available in a large static package.
Other approaches include optical sensors, some of which first increase the apparent size of particles by passing them through a condensing liquid, and which rely on the scattering of laser light to size and count individual particles.
Electrostatic separators and detectors cause particles to become electrically charged, and then select for particle size by manipulating the balance of electrostatic forces and physical inertia. These devices also provide continuous real-time data, but require long columns with high voltages to provide good size resolution.
Airborne particles have also been sorted in a microchannel by dielectrophoresis, but this approach required the introduction of airborne particles into liquid and significant sample preparation before sorting could occur.
The following are some publications in the general field of the invention:                Committee on the Medical Effects of Air Pollutants, “Cardiovascular Disease and Air Pollution,” UK Department of Health, 2006.        C. A. Pope, R. T. Burnett, M. J. Thun, E. E. Calle, D. Krewski, K. Ito and G. D. Thursten, “Lung cancer, cardiopulmonary mortality, and long-term exposure to fine particulate air pollution,” J. Am. Med Assoc., vol. 287, pp. 1132, 2002.        M. L. Bell and D. L. Davis, “Reassessment of the lethal London fog of 1952: novel indicators of acute and chronic consequences of acute exposure to air pollution.        “Environ. Health Perspect., vol. 109, pp. 389-394, 2001. B. Ostro, “Assessing the environmental burden of disease at national and local levels,” WHO Environmental Burden of Disease Series, no. 5, vol. Geneva, World Health Organization, 2004.        U.S. Environmental Protection Agency, “Fine Particle (PM2.5) Designations: Frequent Questions,” vol. 2010, 2008.        World Health Organization, “WHO Air quality guidelines for particulate matter, ozone, nitrogen dioxide and sulfur dioxide: Global update 2005,” 2005.        W. H. Walton and J. H. Vincent, “Aerosol Instrumentation in Occupational Hygiene: An Historical Perspective,” Aerosol Science and Technology, vol. 28, pp. 417, 1998.        B. Chua, A. S. Wexler, N. C. Tien, D. A. Niemeier and B. A. Holmen, “Electrical Mobility Separation of Airborne Particles Using Integrated Microfabricated Corona Ionizer and Separator Electrodes,” Microelectromechanical Systems, Journal of, vol. 18, pp. 4-13, 2009.        H. S. Moon, Y. W. Nam, J. C. Park and H. I. Jung, “Continuous microfluidic airborne bacteria separation using dielectrophoresis,” pp. 2038, 2009.        Jeonggi Seo et al., Membraneless microseparation by asymmetry in curvilinear laminar flows, Journal of Chromatography A, 1162 (2007) 126-131;        Daniel R. Gossett and Dino Di Carlo, Particle Focusing Mechanisms in Curving Confined Flows, Anal. Chem. 2009, 81, 8459-8465;        Shinichi Ookawara et al., Feasibility study on concentration of slurry and classification of contained particles by microchannel, Chemical Engineering Journal 101 (2004) 171-178;        Nobuo Oozeki et al., Characterization of Microseparator/Classifier with a Simple Arc Microchannel, AIChE Journal, January 2009 Vol. 55, No. 1.        
Some patent documents in the field of the invention are:                LeVot et al. US 2009/0283456.        
There is a need for practical and cost-effective methods and apparatus for detecting and/or measuring particles entrained in fluid, such as air, for example. Some applications require such apparatus and methods that are compact and readily portable.